This invention relates to a method of manufacture of MIS (Metal Insulator Semiconductor) and MOS (Metal Oxide Semiconductor) FET (Field Effect Transistor) devices, and more particularly to a method of manufacture of Complementary MIS FET (CMIS) and Complementary MOS FET (CMOS) devices with SiGe gates.
CMIS and CMOS devices include NFET and PFET devices in doped regions in a substrate and counterdoped regions in the substrate respectively. Frequently the region in the substrate in which the PFET devices are formed comprises a counterdoped well in the substrate.
The metal part of a MISFET/MOSFET device is a conductor known as a gate electrode, hereinafter often referred to as gate. The gate electrode is composed of a material such as doped polysilicon or a metal conductor formed above the insulator of the MISFET generally referred to as the gate dielectric layer in a MISFET or gate oxide layer in a MOSFET. The gate electrode is part of a gate electrode stack that includes the gate dielectric layer which is supported on a semiconductor layer or substrate. A channel region is formed in the substrate below the gate dielectric.
A CMIS device or CMOS device includes both NFET (N-channel) devices (with source/drain regions doped with N-type dopant) devices and PFET (P-channel) devices.
A pair of source/drain regions are formed in the substrate juxtaposed with the channel region, generally aligned with the sidewalls of the gate electrode stack. The insulator of such a device is a gate dielectric which separates the gate from the semiconductor substrate upon which the gate dielectric and the gate are formed. In other words, a MIS FET or a MOS FET is a field effect transistor (FET) with a gate formed over an insulator known as a gate dielectric layer which is interposed between a channel region in the semiconductor substrate and the gate. When the gate insulator or gate dielectric insulator of a MISFET is an oxide (typically silicon oxide) the device is known as a Metal Oxide Semiconductor FET (MOSFET) device.
MISFET or MOSFET devices are typically created on the surface of a substrate after either a P-type or a N-type impurity has been implanted in the surface of the substrate, creating wells in this surface of either P-type or N-type conductivity. NMIS or NMOS devices (also referred to as n-channel devices) are, after that, created on the surface of a P-type well. In like manner, PMIS or PMOS devices (also referred to as p-channel devices) are created on the surface of an N-type well.
After the gate has been created, Lightly Doped Drain/Source (LDD/LDS) regions, commonly referred to as LDD regions, are typically implanted in the surface of the substrate, self aligned with the gate, whereby N-type impurities are used for the LDD regions of NMIS/NMOS devices and P-type impurities are used for the LDD regions of PMIS/PMOS devices, which are self aligned with the gates. After this, each of the gates is isolated by the formation of gate spacers on the sidewalls of the gates. This is followed by forming the source and drain regions of the gates which are self aligned with the spacers.
For the source/drain implants the same type impurities are used as have been used for the LDD implants. The difference between the LDD implants and the source/drain implants is that the source/drain implants are typically performed at higher implant energy and dosage that the LDD implants. In this manner the P-type implants (for PMIS/PMOS devices) of the source/drain regions (PS/D) and the n-type implants (for NMIS/NMOS devices) of the source/drain regions (NS/D) penetrate deeper into the surface of the substrate than the corresponding P-type (PLDD) and N-type (NLDD) implants for the LDD regions.
U.S. Pat. No. 6,524,902 of Rhee et al. entitled “Method of Manufacturing CMOS Semiconductor Device” points out that in the fabrication of a CMOS device, boron is doped or implanted into a polysilicon gate layer to form gates of PMOS transistors. The impurity implantation of the p-type doping impurity, e.g. boron, is often carried out along with the formation of the source/drain regions by an ion implantation process. The problem is that where boron is used as a dopant in the impurity implantation, it may diffuse and escape into P-channels through a thin gate insulating layer, unless it is insufficiently planted or activated; and the problem is more serious because the gate insulating layer is very thin. If boron ions escape from the gate in the impurity implantation, boron concentration in the gate near to the gate insulating layer declines and the result is the problem of the Poly-Gate Depletion Effect (PDE). The manufacturing process described by Rhee et al involves blanket deposition of a polysilicon-SiGe layer, followed by formation of additional silicon cap layer formed over the polysilicon SiGe layer. Then phosphorus ions are implanted selectively into an NMOS region, but not into the PMOS region which is covered by an ion implantation mask on the silicon cap layer to cover at least one PMOS transistor region. During a subsequent thermal annealing step, the implanted N-type impurities (phosphorus ions) in the NMOS region enhance diffusion of Ge atoms into the silicon cap layer of the NMOS region, since the annealing step drives Ge atoms in both the downward and upward directions more freely as a result of the ion implantation step. The result is that a desired germanium dopant profile is produced in the SiGe layer and in the silicon layer in the NMOS transistor region, while germanium is substantially prevented from diffusing into the silicon layer in the PMOS transistor region maintaining a high level of Ge near the gate oxide.
The Abstract of an article by E. J. Stewart, M. S. Carroll, and James C. Sturm entitled “Suppression of Boron Penetration in P-Channel MOSFETs Using Polycrystalline Si1-x-yGexCy Gate Layers” in IEEE Electron Device Letters, VOL. 22, NO. 12, (December 2001) 574–576, states that “Boron penetration through thin gate oxides in p-channel MOSFETs with heavily boron-doped gates causes undesirable positive threshold voltage shifts. P-channel MOS-FETs with polycrystalline Si1-x-yGexCy gate layers at the gate-oxide interface show substantially reduced boron penetration and increased threshold voltage stability compared to devices with all poly Si gates or with poly Si1-x-yGex gate layers. Boron accumulates in the poly Si1-X-YGeXCY layers in the gate, with less boron entering the gate oxide and substrate. The boron in the poly Si1-X-YGeXCY appears to be electrically active, providing similar device performance compared to the poly Si or poly Si1-X-YGeXCY gated devices.”
E. J. Stewart, M. S. Carroll, and James C. Sturm entitled “Boron Segregation and electrical properties in polycrystalline Si1-X-YCY and Si1-YCY Layers” in Journal of Applied Physics, VOL. 95, Number 8, (15 Apr. 2004) pp. 4029–4035 “reports strong boron segregation to polycrystalline Si1-x-yGexCy from polysilicon during thermal anneals . . . . ” The article also states “Conventional p-channel MOSFETs with heavily boron-doped polysilicon gates can suffer from voltage instabilities, caused by diffusion from the gate through the gate oxide and into the channel during post implant anneal. Devices with polycrystalline Si1-X-YGeXCY layers in the gate have less boron penetration, and greater threshold voltage stability than devices with polycrystalline Si or Si1-XGeX gate layers.”
U.S. patent application Ser. No. 2004/021743 A1 of Chu entitled “High Performance FET Devices and Methods Therefor” describes a method of fabrication of FET devices in which dopant impurities are prevented from diffusing through the gate insulator. The structure comprises a Si:C, or SiGe:C, layer which is sandwiched between the gate insulator and a layer which is doped with impurities in order to provide a preselected workfunction. The Chu application states that “As the gate insulator is thinned, as dictated by the requirements of ever smaller devices, there is the problem of the doping impurities penetrating the gate insulator, typically an SiO2 layer. For the sake of optimal device design, the gate typically is made of polysilicon, which is doped the same conductivity type as the device itself. With such doping the resultant workfunction of the gate with respect to the channel region of the device allows for the threshold of the device to be optimally set. Accordingly, N-type devices are in need of N-doped gates, and P-type devices are in need of P-doped gates. During the high temperatures of device manufacturing, the gate-doping species, most problematically boron, (B), but others like phosphorus (P) as well, readily penetrates the thin gate insulator and the result is that the gate-doping species destroys the device. The gate insulator in modem high performance devices typically needs to be less than about 3 nm thick. Preventing this dopant penetration would be an important step in achieving thinner gate insulators.”
U.S. Patent Publication 2004/0067631 of Bu et al. entitled “Reduction of Seed Layer Roughness for Use in Forming SiGe Gate Electrode” describes how to deposit a “seed” layer to facilitate deposition of a SiGe layer. The method provides for fabricating layers for use in formation of a silicon germanium (SiGe) gate starting with a substrate having a first surface. Then a gate dielectric layer is formed overlying the first surface of the substrate. Next the gate dielectric layer is treated with a gaseous medium to modify a surface characteristic of the gate dielectric. Then, a seed layer is formed overlying the treated gate dielectric thereby mitigating roughness of the seed layer. Then a SiGe layer is formed overlying the seed layer, so that the germanium (Ge) interdiffuses into the seed layer.
U.S. Pat. No. 6,709,912 of Ang et al. entitled “Dual Si-Ge Polysilicon Gate with Different Ge Concentrations for CMOS Device Optimization” describes a method for increasing the amount of Ge over a PMOS region through further implanting and laser annealing. Dual Si-Ge polysilicon gates are formed with different Ge concentrations in the fabrication of an integrated circuit device. An NMOS active area and a PMOS active area of a substrate separated by an isolation region are provided. A gate oxide layer is grown overlying the substrate in both active areas. A polycrystalline silicon-germanium (Si-Ge) layer is deposited overlying the gate oxide layer with the polycrystalline SiGe layer having a first Ge concentration. The NMOS active area is blocked while exposing the PMOS active area and performing successive cycles of Ge plasma doping and laser annealing into the PMOS active area to achieve a second Ge concentration higher than the first Ge concentration. Then, the polycrystalline Si-Ge layer is patterned to form a gate in both active areas. The gate in the PMOS active area has a higher Ge concentration than the gate in the NMOS active area. That completes formation of the dual Si-Ge polysilicon gates with different Ge concentrations in the fabrication of an integrated circuit device.
Referring again to U.S. Pat. No. 6,524,902 of Rhee et al entitled “Method of Manufacturing CMOS Semiconductor Device” the steps of the method are described next. Form a gate insulating layer on a substrate and a SiGe layer having Ge content of more than 20% on the gate insulating layer. Then form a silicon layer on the SiGe layer and form an ion implantation mask on the silicon layer to cover at least one PMOS transistor region. Perform a n-type impurity ion implantation process on at least one NMOS transistor region of the substrate having the ion implantation mask. Perform a diffusion and annealing process on the substrate in which n-type impurities are implanted to diffuse germanium into the silicon layer in the NMOS transistor region to produce a desired germanium dopant profile in the SiGe layer and the silicon layer in the NMOS transistor region, while germanium is substantially prevented from diffusing into the silicon layer in the PMOS transistor region. Then form a gate pattern for PMOS and NMOS transistors by patterning the silicon layer and the SiGe layer. The result of the process is that in the NMOS region where the presence of Ge is unwanted, some SiGe remains present.
In U.S. Pat. No. 6,730,588 of Schinella entitled “Selective deposition of SiGe” a dielectric layer is deposited on a semiconductor wafer. Then a thin silicon layer of amorphous silicon or polycrystalline silicon is deposited on the dielectric layer. A mask is formed over the thin silicon layer, which is used during etching to form a silicon nucleation layer and to expose portions of the dielectric layer. A self-aligned gate is formed by depositing a silicon germanium conducting film on the silicon nucleation layer using a material which selectively deposits on the nucleation layer and which falls to deposit on the exposed portions of the dielectric layer. Then a metal layer is deposited on the top surface of the silicon germanium conducting film. The method employed is very different from that of the present invention.
It is well known that boron activation is superior in poly-SiGe compared to that in polysilicon alone. Thus PFETs with SiGe gates can exhibit a less significant polysilicon depletion effect. The result is that there is a thinner electrical equivalent gate oxide thickness (or inversion thickness), which is beneficial for PFET drive current. In this context, thinner thickness means larger gate capacitance which directly is related to drive current.
However, N type dopant activation in the NFET region may be poorer in the presence of SiGe as evidenced by larger inversion thickness (Tinv) in NFETs with SiGe gates. As employed herein, the term “inversion thickness” (Tinv) is the electrically measured oxide thickness when a MOS device is in inversion. When Tinv is less (thinner) the gate capacitance is greater, leading to a higher drive current.
A serious problem is the “poly depletion effect” which reduces gate capacitance, with the reduced gate capacitance having the disadvantage of reducing device drive current.